No matter how well-designed, well-researched, and well-implemented any given product or experience is, it will never work for 100% of people. This is true for health interventions, consumer products, financial services, you name it. And while it sounds pessimistic to say that, the reason why is both obvious and (at least to me) interesting: Everybody is different.
Psychology is not a magic bullet for creating engaging or effective experiences. I think of it as a way to apply science to improve the odds that whatever you create will work for its target audience. A key aspect of any design process is understanding the end users, ideally through research, so that your design can be targeted to their needs and wants. You may be lucky enough to work on a project that hits a huge swathe of the population–for example, products like Netflix or Amazon are used by all sorts of people–but that swathe will never be 100%.
I was reminded of this reading Flow: The Psychology of Optimal Experience by Mihaly Csikszentmihalyi. He talks about how work can be reconsidered to emphasize positive challenges, turning labor into a source of enjoyment and even happiness. But he cautions:
It would be erroneous to expect that if all jobs were constructed like games, everyone would enjoy them. Even the most favorable external conditions do not guarantee that a person will be in flow. Because optimal experience depends on a subjective evaluation of what the possibilities for action are, and of one’s own capacities, it happens quite often that an individual will be discontented with even a potentially great job.
All of which brings me back to another theme of my research and approach: You have to be authentically yourself if you want to be happy and effective in your life. Everybody is different. As designers, we need to know that nothing we build will work for everyone. As users, we need to know that not every product will work for us. The secret is to know yourself and your user well enough to make the right match.